Simultaneous X-ray and radio observations of GX 5 - 1

Simultaneous X-ray (Ginga) and radio (VLA) observations of the Z-type low-mass X-ray binary GX 5 - 1 were made during September 1-6, 1989. The radio flux of GX 5 - 1 was less than 1 mJy when the X-ray source was in the horizontal branch. In the normal branch, the radio flux increased up to about 6 mJy, and its variations were anticorrelated with those of the X-ray flux during time intervals of tens of minutes. This radio behavior is opposite to the X-ray/radio correlation found before in the Z sources GX 17 + 2, Cyg X - 2, and Sco X - 1. The radio properties of GX 5 - 1 are different from those of these three Z sources, at 99.5 percent and 96.5 percent levels of confidence for the horizontal branch and the normal/flaring branches, respectively.